1. Field of the Invention
This invention relates to a method for the production of a sheetlike fiber-reinforced thermosetting resin molding material by the deaeration of a mixture of a liquid thermosetting resin composition with reinforcing fibers for impregnation of the fibers with the resin composition. More particularly, it relates to a method which effects thorough deaeration of the mixture and thorough impregnation of the fibers with the resin composition by exertion of a uniform pressing force on the mixture throughout the whole volume thereof without reference to possible variation in wall thickness of the mixture and which, therefore, produces a molding material of homogeneous quality.
2. Description of the Prior Art
Generally, the hot press quality fiber-reinforced thermosetting resin molding material is obtained by mixing a viscous liquid resin composition having such additives as filler, mold release agent, tackifier, and coloring agent in a resin solution such as of an unsaturated polyester resin with reinforcing fibers such as glass fibers and causing the reinforcing fibers to be impregnated with the resin solution. It is put to use for molding in a state free from excessive viscidity and flexible enough to permit easy handling.
The sheet molding compound (SMC) method may be cited as a representative of the conventional methods adopted for the production molding materials supplied in the form of sheet. The SMC method produces the molding material by applying a liquid resin composition to two vertically opposed plastic films of a polyolefin, causing reinforcing fibers of a small length cut as from a glass roving to be sprayed between the opposed plastic films, joining the vertically opposed plastic films thereby forming a laminate composed of a mixture of the resin composition and the reinforcing fibers, deaerating the laminate by rolling thereby ensuring thorough impregnation of the reinforcing fibers with the resin composition and consequent conversion of the laminate into a sheet, and winding the produced sheet into a roll.
One of the apparatuses heretofore employed for the impregnation involved in the SMC method is constructed as illustrated in FIG. 1. In this impregnation apparatus, a mixture 2 of a resin composition and reinforcing fibers supported between two vertically opposed films 1a and 1b is exposed to the pressing force to be exerted thereon by a plurality of impregnation-deaeration rollers 9, 9, . . . and 10, 10 . . . disposed in rows across a pair of vertically opposed endless belts 3a and 3b. While the mixture of the resin composition and the reinforcing fibers is passing between the endless belts 3a and 3b, the bubbles entrapped in the mixture are sequentially pushed out and the resin composition is forced into the gaps between the reinforcing fibers to effect the impregnation of the reinforcing fibers with the resin composition.
In the case of a molding material of insufficient impregnation and deaeration, products obtained by forming this molding material are liable to entail such defects as molding cavities, pinholes, and prominent fiber patterns. It is known that uneven impregnation aggravates the dispersion of product strength.
Various devices have been developed for the SMC method to attain thorough impregnation and deaeraton and allow manufacture of flawless formed products of high strength. It is nevertheless difficult to product SMC articles of large thickness. Generally, the largest thickness of sheet in which thorough impregnation and deaeration is attained is roughly on the order 2 to 3 mm. For the resin composition, such high viscosity as exceeding 1,000 poises is not suitable. Generally, the resin composition used for the SMC method has viscosity approximately in the range of 100 to 200 poises. Even in the case of a sheet material of such small thickness as used by the SMC method, the products of the SMC method are liable to suffer from excessive distribution of resin composition in the end parts thereof because the pressing force generated by the impregnation-deaeration rollers frequently induce the phenomenon of exudation of resin composition in the direction of width of the film. Further, the sheet which is taken up in a roll after the step of impregnation and deaeration is liable to entail the transfer of resin composition in the direction of width of the film under the tension of the film during the course of the winding.
For the purpose of preventing the exudation of resin composition during the step of winding, the practice of folding the lateral edges of the two vertically opposed films perpendicularly to the direction of travel of the sheet and joining the folded lateral edges while the sheet is in the process of transfer from the step of impregnation and deaeration to the step of winding has been in vogue. This practice, however, is not free from the heterogeneity of SMC owing to the transfer of the resin composition which occurs during the step of impregnation and deaeration.
Japanese Patent Laid-Open SHO 54 (1979)-150,476, for the purpose of providing efficient impregnation and deaeration for a mixture of a resin composition and reinforcing fibers, proposes an impregnation apparatus improved so as to compress the mixture and, at the same time, impart a shearing effect thereto. In this patent laid-open, there is a mention to the effect that by exerting the pressing force continuously and, at the same time, using a mesh belt, the relative movement of reinforcing fibers in the mixture is repressed and the transfer of the resin composition in the direction of width of the film is precluded because the resin composition enters the pockets formed in the openings of the meshed belt. Even with such kind of impregnation apparatus, it is difficult to have reinforcing fibers to be impregnated with a highly viscous resin composition whose liquid viscosity exceeds 1,000 poises. Thus, the impregnation apparatus produces a desired molding material with difficulty.
As a way of amply impregnating reinforcing fibers with a highly viscous resin composition without inflicting an injury to the reinforcing fibers, the method disclosed in U.S. Pat. No. 4,702,872 may be cited. In the sheet produced by this method, however, an effort to increase the thickness of this sheet entails a gradient variation of wall thickness in cross section of the edge parts of the sheet. With the conventional impregnation apparatus, therefore, it is difficult to exert a uniform pressing force to the edge parts of the sheet. Consequently a homogeneous sheet of large thickness with a satisfactory state of impregnation is not obtained.
An object of this invention, therefore, is to provide a novel method for the production of a fiber-reinforced thermosetting resin molding material.
Another object of this invention, in the production of a sheet like fiber-reinforced thermosetting resin composition by the impregnation of reinforcing fibers with a liquid thermosetting resin composition, is to provide a method for the production of a homogeneous molding material in a state of satisfactory state of impregnation, which method allows a pressing force to be exerted on the sheet uniformly throughout the entire surface including the edge parts without reference to the viscosity of the resin composition and the thickness of the sheet and ensures repression of the transfer of the resin composition.